This invention relates to a frequency-modulation stereophonic receiver, and more particularly to an FM stereophonic receiver free from noticeable noise and distortion in reproduced audio output when automatically switched over between stereophonic and monophonic modes depending on variations in field strength of FM carrier wave to be received. This invention is particularly useful for automotive applications where the received broadcast signal is frequently subject to variations in field strength or signal level.
A problem heretofore encountered with the conventional stereophonic FM receivers, particularly mounted in automobiles, resides in the fact that frequent intensity variations in the received broadcast signal invariably results in insufficient channel separation with the attendant generation of uncomfortable noise in the audio output.
It is known that the signal-to-noise ratio of a received stereophonic FM signal is theoretically about 20 dB lower than that of monophonic FM signals and therefore stereophonic reproduction under poor receptivity would result conditions which tend to generate noise of a noticeable degree.
The conventional practice for reducing this noise problem is to automatically or manually switch over the receiver from the stereophonic reception mode to the monophonic when the receptivity is so poor that noise increases to a noticeable degree. However, such a system tends to produce crosstalk between the left and right channels upon the switchover and this results in distortion in audio output signals or so-called "pop" noise. This problem is considerable among others with the FM stereophonic receiver mounted in an automobile.
It is therefore a primary object of this invention to provide an FM stereophonic receiver which reduces noise when the receiver is switched over between stereophonic and monophonic modes in respnse to different levels of received broadcast signals.
It is another object of this invention to provide an FM stereophonic receiver having an improved FM demodulator for increasing signal-to-noise ratio under poor receptivity of FM stereophonic signal.
It is another object of this invention to provide an FM stereophonic receiver having an improved FM demodulator suitable for being manufactured by conventional integrated circuit technique.
Still another object of this invention is to provide an FM stereophonic receiver having an improved demodulator which maintains DC components, on which FM stereophonic composite signals are superimposed, at a constant level at all times for overcoming the aforementioned noise problem.